1990 S Lincoln Memorial Cent Small Cents : No S Mint Mark

Copper Plated Zinc Penny

What this coin looks like (obverse, reverse, mint mark location, special features, etc.):

Some 1990 S Lincoln Memorial Cents (Pennies) did not receive an S mint mark on the coin. These are extremely rare and valuable. The San Francisco Mint produced only Proof coins and this error coin is also a Proof coin so we know it came from the San Francisco Mint but the S mint mark is absent.